1. Field of the Invention
The present invention relates to an apparatus for and a method of controlling a plant.
2. Description of the Related Art
The applicant of the present application has proposed a technique for controlling the air-fuel ratio of an air-fuel mixture to be combusted in an internal combustion engine, and hence the air-fuel ratio of an exhaust gas that enters a catalytic converter (hereinafter referred to as xe2x80x9cupstream-of-catalyst air-fuel ratioxe2x80x9d) in order to converge the output of an exhaust gas sensor (oxygen concentration sensor) to a given target value (constant value), the oxygen concentration sensor being disposed downstream of the catalytic converter for detecting the concentration of a certain component, e.g., the concentration of oxygen, in the exhaust gas that has passed through the catalytic converter, in order to enable the catalytic converter disposed in the exhaust passage of the internal combustion engine to perform its desired exhaust gas purifying capability, as disclosed in Japanese laid-open patent publication No. 11-93740 and U.S. Pat. No. 6,079,205. The upstream-of-catalyst air-fuel ratio is specifically the air-fuel ratio of a combusted air-fuel mixture that enters the catalytic converter, as recognized from the concentration of oxygen in the exhaust gas.
According to the disclosed system, an exhaust system ranging from an upstream side of the catalytic converter to the oxygen concentration sensor disposed downstream of the catalytic converter is used a system to be controlled. A manipulation variable for determining the upstream-of-catalyst air-fuel ratio as an input quantity for the exhaust system, e.g., a target air-fuel ratio for the exhaust gas, is successively generated in order to converge the output of the oxygen concentration sensor as an output variable from the exhaust system to the target value. By manipulating the air-fuel ratio of the air-fuel mixture combusted in the internal combustion engine depending on the target air-fuel ratio, the upstream-of-catalyst air-fuel ratio is manipulated into the target air-fuel ratio, and hence the output of the oxygen concentration sensor is converged to the target value.
The exhaust system has a relatively long dead time owing to the catalytic converter included in the exhaust system. While the internal combustion engine is operating in a low rotational speed range, e.g., is idling, the dead time of a system for generating the upstream-of-catalyst air-fuel ratio from the target air-fuel ratio, which system includes the internal combustion engine and will be referred to as xe2x80x9cair-fuel ratio manipulating systemxe2x80x9d) is also relatively long. These dead times tend to adversely affect the process of converging the output of the oxygen concentration sensor to the target value. According to the above technique, therefore, data representing an estimated value for the output of the oxygen concentration sensor after the dead time of the exhaust system or the sum of the dead time of the exhaust system and the dead time of the air-fuel ratio manipulating system is sequentially generated according to an algorithm which is constructed based on a predetermined model of the exhaust system. The target air-fuel ratio is generated using the above estimated value. The target air-fuel ratio is generated according to a sliding mode control process (specifically, an adaptive sliding mode control process) which is one type of feedback control process.
The air-fuel ratio of the exhaust gas while the output of the oxygen concentration sensor is being converged to the target value is an air-fuel ratio close to a stoichiometric air-fuel ratio.
According to the above technique, the control process of converging the output of the oxygen concentration sensor to the target value can stably be carried out while compensating for the effect of the dead times of the exhaust system and the air-fuel ratio manipulating system, and the good purifying capability of the catalytic converter can be maintained irrespectively of a degraded state of the catalytic converter.
In the above technique, if the exhaust system is regarded as a plant, then the internal combustion engine can be regarded as an actuator for generating the upstream-of-catalyst air-fuel ratio as an input to the plant and the oxygen concentration sensor as a detecting means for detecting the concentration of oxygen as an output of the plant.
Generally, internal combustion engines mounted on automobiles or the like are not always operated at an air-fuel ratio close to a stoichiometric air-fuel ratio in a mode referred to as xe2x80x9cstoichiometric operation modexe2x80x9d. Depending on the operating conditions, the internal combustion engine may be operated with the fuel supply being cut off or operated in a lean air-fuel ratio range in a mode referred to as xe2x80x9clean operation modexe2x80x9d. The control process of converging the output of the oxygen concentration sensor to the target value is carried out in the stoichiometric operation mode.
The output of the oxygen concentration sensor exhibits substantially linear characteristics with respect to the concentration of oxygen in a range close to the target value, i.e., in an air-fuel ratio range close to a stoichiometric air-fuel ratio. However, the output of the oxygen concentration sensor is nonlinear with respect to the concentration of oxygen out of the range close to the target value (see the solid-line curve xe2x80x9caxe2x80x9d in FIG. 2 of the accompanying drawings). Therefore, the output of the oxygen concentration sensor is nonlinear with respect to the concentration of oxygen when the internal combustion engine is operated with the fuel supply being cut off or operated in the stoichiometric operation mode immediately after the lean operation mode.
However, when the output of the oxygen concentration sensor varies in the nonlinear range, according to the above technique, the accuracy of the estimated value for the output of the oxygen concentration sensor tends to be lowered, and it is difficult to keep the accuracy of the estimated value at the same level as when the output of the oxygen concentration sensor is in the linear range. In addition, the catalyst has different responses when the catalyst exhibits a reducing action based on its chemical reaction, i.e., when the air-fuel ratio changes from a lean side to a rich side, and when the catalyst exhibits an oxidizing action based on its chemical reaction, i.e., when the air-fuel ratio changes from a rich side to a lean side, and the nonlinearity of the oxygen concentration sensor also increases with the different responses of the catalyst. The sliding mode control process (specifically, the adaptive sliding mode control process) which is a stable control process for generating the target air-fuel ratio may be used to prevent the stability of the process of controlling the output of the oxygen concentration sensor from being impaired. However, the quick response of the control process of converging the output of the oxygen concentration sensor may be impaired due to the reduction in the accuracy of the estimated value for the output of the oxygen concentration sensor.
It is therefore an object of the present invention to provide an apparatus for and a method of controlling a plant so as to be able to perform, with a highly quick response, a control process of converging an output of a detecting means for detecting the output of the plant to a predetermined target value irrespective of the output state of the detecting means even if the output of the detecting means is nonlinear.
Another object of the present invention is to provide an apparatus for and a method of controlling a plant so as to be able to perform, with a highly quick response, a control process of converging an output of a an exhaust gas sensor (detecting means) such as an oxygen concentration sensor disposed downstream of a catalytic converter in the exhaust passage of an internal combustion engine to a predetermined target value irrespective of the output state of the exhaust gas sensor for thereby increasing the exhaust gas purifying capability of the catalytic converter, even if the output of the exhaust gas sensor is nonlinear.
To achieve the above objects, an apparatus for controlling a plant according to the present invention is available in a first aspect and a second aspect. According to the first aspect, an apparatus for controlling a plant for generating an output from an input applied thereto basically comprises detecting means for detecting the output of the plant, and manipulation variable generating means for sequentially generating a manipulation variable for manipulating the input to the plant in order to converge an output of the detecting means to a predetermined target value.
To achieve the above objects, a method of controlling a plant according to the present invention is available in a first aspect and a second aspect. According to the first aspect, a method of controlling a plant for generating an output from an input applied thereto comprises the steps of detecting the output of the plant with detecting means, and sequentially generating a manipulation variable for manipulating the input to the plant in order to converge an output of the detecting means to a predetermined target value;
The apparatus according to the first aspect performs a control process of converging the output of the detecting means to the target value while compensating for the effect of a dead time of the plant. The apparatus has a plurality of estimating means for sequentially generating data of estimated values of the output of the detecting means after a dead time of the plant, according to respective different algorithms using at least data of the output of the detecting means, the manipulation variable generating means comprising means for generating the manipulation variable using selectively the estimated value represented by either one of the data generated by the plurality of estimating means under a given condition or using an estimated value which comprises the estimated values represented by the data generated by the plurality of estimating means and combined with each other under a given condition.
The method according to the first aspect comprises the steps of sequentially generating data of estimated values of the output of the detecting means after a dead time of the plant, according to a plurality of different estimating algorithms using at least data of the output of the detecting means, and generating the manipulation variable using selectively the estimated value represented by either one of the data generated by the plurality of estimating algorithms under a given condition or using an estimated value which comprises the estimated values represented by the data generated by the plurality of estimating algorithms and combined with each other under a given condition.
With the apparatus and method according to the first aspect, since the plurality of estimating means or estimating algorithms generate the data of the estimated values of the output of the detecting means after the dead time according to the respective different algorithms, it is possible to generate data representing a plurality of estimated values to match a plurality of types of output states (depending on the output state of the plant) of the detecting means. Therefore, even if the output of the detecting means is nonlinear, when the estimated value represented by either one of the data generated by the plurality of estimating means or estimating algorithms is selected under a given condition (e.g., a condition relative to an output state of the detecting means or data correlated thereto) or a combined estimated value produced by combining the estimated values under the given condition is determined, the selected estimated value or the combined estimated value is highly accurate as an estimated value of the dead time of the plant.
The manipulation variable thus generated using the selected estimated value or the combined estimated value is appropriate for use in converging the output of the detecting means to the target value while compensating for the effect of the dead time of the plant irrespectively of the output state of the detecting means or the output state of the plant. As a result, the quick response of the control process of converging the output of the detecting means to the target value can be increased irrespectively of the output state of the detecting means.
The apparatus for and the method of controlling the plant according to the second aspect of the present invention has, in addition to the basic components of the first aspect, an actuator for generating the input to the plant and actuator control means for controlling the actuator depending on the manipulation variable to manipulate the input to the plant.
The apparatus according to the second aspect performs a control process of converging the output of the detecting means to the target value while compensating for the effect of a total dead time which is the sum of a dead time of the plant and a dead time of an input manipulating system comprising the actuator control means and the actuator. The apparatus has a plurality of estimating means for sequentially generating data of estimated values of the output of the detecting means after the total dead time, according to respective different algorithms using at least data of the output of the detecting means, the manipulation variable generating means comprising means for generating the manipulation variable using selectively the estimated value represented by either one of the data generated by the plurality of estimating means under a given condition or using an estimated value which comprises the estimated values represented by the data generated by the plurality of estimating means and combined with each other under the given condition.
The method according to the second aspect comprises the steps of sequentially generating data of estimated values of the output of the detecting means after a total dead time which is the sum of a dead time of the plant and a dead time of an input manipulating system comprising the actuator control means and the actuator, according to a plurality of different estimating algorithms using at least data of the output of the detecting means, and generating the manipulation variable using selectively the estimated value represented by either one of the data generated by the plurality of estimating algorithms under a given condition or using an estimated value which comprises the estimated values represented by the data generated by the plurality of estimating algorithms and combined with each other under the given condition.
With the apparatus and method according to the first aspect, since the plurality of estimating means or estimating algorithms generate the data of the estimated values of the output of the detecting means after the total dead time according to the respective different algorithms, it is possible to generate data representing a plurality of estimated values to match a plurality of types of output states (depending on the output state of the plant) of the detecting means. Therefore, as with the first aspect, even if the output of the detecting means is nonlinear, when the estimated value represented by either one of the data generated by the plurality of estimating means or estimating algorithms is selected under a given condition (e.g., a condition relative to an output state of the detecting means or data correlated thereto) or a combined estimated value produced by combining the estimated values under the given condition is determined, the selected estimated value or the combined estimated value is highly accurate as an estimated value of the total dead time which is the sum of the dead time of the plant and the dead time of the input manipulating system (the system for generating the input to the plant from the manipulation variable) comprising the actuator control means and the actuator.
The manipulation variable thus generated using the selected estimated value or the combined estimated value is appropriate for use in converging the output of the detecting means to the target value while compensating for the effect of the total dead time irrespectively of the output state of the detecting means or the output state of the plant. As a result, as with the first aspect, the response of the control process of converging the output of the detecting means to the target value can be increased irrespectively of the output state of the detecting means.
In the apparatus for controlling the plant, the manipulation variable may be a target input to the plant, a corrective quantity for the operation of the actuator, or the like. If the manipulation variable is a target input to the plant, then it is preferable to provide the detecting means for detecting the input to the plant and manipulate the input to the plant according to a feedback control process in order to converge the output of the detecting means (the detected value of the input to the plant) to the target input. The manipulation variable generating means for generating the manipulation variable using the estimated value may generate the manipulation variable according to a feedback control process in order to converge the estimated value to the target value for the output of the detecting means for thereby generating the manipulation variable capable of appropriately compensating for the effect of the dead time of the plant or the total dead time.
According to the first and second aspects of the present invention, the plant may comprise an exhaust system ranging from a position upstream of a catalytic converter, which is disposed in an exhaust passage of an internal combustion engine for purifying an exhaust gas flowing in the exhaust passage, to a position downstream of the catalytic converter, and including the catalytic converter. The input to the plant comprises an air-fuel ratio of the exhaust gas generated by the internal combustion engine and entering the catalytic converter, and the output from the plant comprises the concentration of a component of the exhaust gas which has passed through the catalytic converter. In the second aspect, the internal combustion engine functions as the actuator.
Since the output of the exhaust system as the plant represents the concentration of a component of the exhaust gas which has passed through the catalytic converter, the detecting means is an exhaust gas sensor disposed downstream of the catalytic converter for detecting the concentration of the component of the exhaust gas. The manipulation variable for manipulating the air-fuel ratio of the exhaust gas (the air-fuel ratio of the exhaust gas entering the catalytic converter) as the input to the plant in order to converge the output of the detecting means (exhaust gas sensor) to the target value, and the air-fuel ratio of the exhaust gas entering the catalytic converter is manipulated according to the manipulation variable. For generating the manipulation variable, the dead time of the exhaust system which is represented by the data generated respectively by the plurality of estimating means or estimating algorithms, or the estimated value of the output of the detecting means (exhaust gas sensor) after the total dead time which is the sum of the dead time and the dead time of the input manipulating system is selectively used. Alternatively, a combined estimated value produced by combining these estimated values is used.
Since the quick response of the control process of converging the output of the detecting means to the target value can be increased irrespectively of the output state of the plant or the output state of the detecting means, the quick response of the control process of converging the output of the exhaust gas sensor to the target value can be increased irrespectively of the exhaust gas state at a downstream end of the exhaust system or the output state of the exhaust gas sensor as the detecting means downstream of the catalytic converter. As a result, a desired gas purifying capability of the catalytic converter can appropriately be achieved.
With the plant comprising the exhaust system of the internal combustion engine, the manipulation variable may be a target air-fuel ratio (a target input to the plant) of the exhaust gas entering the catalytic converter or a corrective quantity for the amount of fuel supplied to the internal combustion engine as the actuator. If the manipulation variable is the target air-fuel ratio of the exhaust gas entering the catalytic converter, then it is preferable to provide a detecting means (air-fuel ratio sensor) for detecting the air-fuel ratio of the exhaust gas (an actual input to the plant) upstream of the catalytic converter, and manipulate the air-fuel ratio of an air-fuel mixture combusted by the internal combustion engine according to a feedback control process in order to converge the output of the air-fuel ratio sensor (the detected value of the air-fuel ratio of the exhaust gas) to a target air-fuel ratio. The feedback control process should preferably be performed by a recursive-type controller such as an adaptive controller or the like.
According to the first and second aspects, the apparatus or the step has means for or the step of determining a combined estimated value by weighting and combining the estimated values represented by the data generated by the plurality of estimating means or estimating algorithms. For determining the combined estimated value, weighting coefficients relative to the estimated values of the respective estimating means or estimating algorithms are variably established under the given condition to determine the combined estimated value including the estimated values of the respective estimating means or estimating algorithms. The manipulation variable should preferably be generated using the determined combined estimated value.
By variably establishing the weighting coefficients under the given condition, either one of the estimated values represented by the data generated respectively by the plurality of estimating means or estimating algorithms may be obtained as the combined estimated value (e.g., the weighting coefficient relative to one estimated value is set to xe2x80x9c1xe2x80x9d, and the weighting coefficient relative to another estimated value is set to xe2x80x9c0xe2x80x9d), and an estimated value which is a combination of those estimated values may be obtained as the combined estimated value. Therefore, since estimated values for use in generating the manipulation variable can be selected or combined by setting the weighting coefficients, an algorithm for selecting and combining estimated values can be constructed with ease.
According to the present invention, the manipulation variable generating means can generate the manipulation variable according to any of various feedback control processes. However, it is preferable to generate the manipulation variable according to an adaptive control process or a sliding mode control process.
By generating the manipulation variable according to the adaptive control process, it is possible to generate the manipulation variable depending on the behavioral state of the plant, increasing the quick response of the control process of converging the output of the detecting means to the target value. The sliding mode control process generally has such characteristics that its control stability against disturbances and modeling errors of objects to be controlled is high. By generating the manipulation variable according to the sliding mode control process, even if an error of the estimated value used therein is unexpectedly large due to the effect of disturbances or the like, any unstability of the output of the detecting means due to the large error is minimized, thus increasing the control stability in converging the output of the detecting means to the target value.
The sliding mode control process should particularly preferably be an adaptive sliding mode control process which incorporates a control law known as an adaptive law (adaptive algorithm) for minimizing the effect of a disturbance, in a normal sliding mode control process. The adaptive sliding mode control process will be described briefly below. The sliding mode control process generally employs a switching function constructed of the difference between a controlled variable and a target value thereof, and it is important to converge the value of the switching function to xe2x80x9c0xe2x80x9d. The normal sliding mode control process uses a control law known as a reaching law to converge the value of the switching function to xe2x80x9c0xe2x80x9d. However, when affected by a disturbance, it may be difficult to sufficiently achieve a desired level of stability and quick response of the process of converging the value of the switching function only with the reaching law. The adaptive sliding mode control process uses a control law known as an adaptive law (adaptive algorithm) in addition to the reaching law for converging the value of the switching function to xe2x80x9c0xe2x80x9d while minimizing the effect of the disturbance.
According to the present invention, the given condition which defines estimated values used to generate the manipulation variable, or stated otherwise, the given condition which defines how to select or combine estimated values used to generate the manipulation variable, should preferably be a condition based on the value of the data of the output of the detecting means, for example. The value of the data of the output of the detecting means directly represents the output state of the detecting means. With the given condition being determined based on the value of the data of the output of the detecting means, the condition can be set with ease, and it is possible to select an estimated value matching the output state of the detecting means or obtain a combined estimated value matching the output state of the detecting means.
If the manipulation variable is generated according to the sliding mode control process (including the adaptive sliding mode control process), then the given condition should preferably comprise a combination condition of the value of a given linear function having as variable components time-series data of the output of the detecting means and determined depending on a switching function used in the sliding mode control process, and the value of the data of the output of the detecting means.
Specifically, if the sliding mode control process is used to generate the manipulation variable, then a combination of the value of a given linear function determined depending on a switching function used in the sliding mode control process and having as variable components time-series data of the output of the detecting means, and the value of the data of the output of the detecting means is highly correlated to the output state of the detecting means. Therefore, if this combination condition is used as the given condition, then it is possible to select an estimated value matching the output state of the detecting means or obtain a combined estimated value matching the output state of the detecting means. It is thus possible to appropriately generate the manipulation variable matching the output state of the detecting means, increasing the quick response of the control process of converging the output of the detecting means to the target value.
If the switching function comprises a linear function having as variable components time-series data of the difference between the output of the detecting means and the target value, then the given linear function should preferably comprise a linear function having coefficient values relative to the variable components thereof, the coefficient values being the same as coefficient values relative to the variable components of the switching function.
Using such a linear function, the combination condition which defines how to select or combine estimated values used to generate the manipulation variable can properly be established. The given linear function may be a function of the same form as the switching function.
The combination condition should preferably include a condition as to whether a combination of the value of the linear function and the value of the data of the output of the detecting means is present in a predetermined range on a coordinate plane which has the value of the linear function and the value of the data of the output of the detecting means as coordinate components.
With the above condition included, it is easy to classify and distinguish combinations of the value of the linear function and the value of the data of the output of the detecting means, making it possible to easily and appropriately establish the combination condition.
The algorithms of the estimating means (estimating algorithms) may be constructed depending on the output characteristics of the detecting means and the behavioral characteristics of the output of the plant, and can be selected from various algorithms.
The estimating means or estimating algorithms may be arranged as follows: According to the first aspect which generates the data representing the estimated values after the dead time of the plant, the plurality of estimating means or estimating algorithms comprise first estimating means or a first estimating algorithm for generating the data representing the estimated value according to an algorithm constructed based on a predetermined model of the plant which expresses a behavior of the plant as a system for generating the output of the detecting means from the input via a response delay element and a dead time delay, and second estimating means or a second estimating algorithm for generating the data representing the estimated value according to the algorithm of a fuzzy inference process.
According to the second aspect which generates the data representing the estimated values after the total dead time which is the sum of the dead time of the plant and the dead time of the input manipulating system, the plurality of estimating means or estimating algorithms comprise first estimating means or a first estimating algorithm for generating the data representing the estimated value according to an algorithm constructed based on a predetermined model of the plant which expresses a behavior of the plant as a system for generating the output of the detecting means from the input via a response delay element and a dead time element, and second estimating means or a second estimating algorithm for generating the data representing the estimated value according to the algorithm of a fuzzy inference process.
If the plant comprises an exhaust system including the catalytic converter, then for achieving an optimum gas purifying capability of the catalytic converter, it is preferable to use an oxygen concentration sensor as the detecting means (exhaust gas sensor), and use the target value as a given constant value.
If the plant comprises an exhaust system including the catalytic converter and the detecting means comprises an oxygen concentration sensor, then the plurality of estimating means or estimating algorithms should preferably comprise the first estimating means or the first estimating algorithm, and the second estimating means or the second estimating algorithm.
Specifically, if the plant is the exhaust system and the detecting means is the oxygen concentration sensor, then according to the first aspect, it is possible to appropriately generate the data representing the estimated values after the dead time of the exhaust system in a state where the output of the oxygen concentration sensor varies in a range (close to the target value) in which it varies substantially linearly with respect to the oxygen concentration in the exhaust gas, basically according to the algorithm constructed based on the model of the exhaust system.
According to the second aspect, it is possible to relatively accurately generate the data representing the estimated values after the total dead time in a state where the output of the oxygen concentration sensor varies in a range (close to the target value) in which it varies substantially linearly with respect to the oxygen concentration in the exhaust gas, basically according to the algorithm constructed based on the model of the exhaust system and the model of the input manipulating system (the model in which the input manipulating system is regarded simply as a dead time element).
According to the findings of the inventors of the present invention, in either the first aspect or the second aspect, the data representing the estimated values after the dead time of the exhaust system or the total dead time in a state where the output of the oxygen concentration sensor varies in a range in which it varies nonlinearly with respect to the oxygen concentration in the exhaust gas, can be generated relatively accurately according to the algorithm of a fuzzy inference process.
If the plurality of estimating means or estimating algorithms should preferably comprise the first estimating means or the first estimating algorithm based on the model of the exhaust system, and the second estimating means or the second estimating algorithm based on the fuzzy inference process, the data of the estimated values can appropriately be generated in respective different output states of the oxygen concentration sensor. Using the estimated values selectively or using a combined value of the estimated values, it is possible to generate the manipulation variable which is suitable for converging the output of the oxygen concentration sensor to the target value regardless of the output state of the oxygen concentration sensor.
In the apparatus for and the method of controlling the exhaust system (plant) with the oxygen concentration sensor used as the detecting means, if the plurality of estimating means or estimating algorithms comprise the first and second estimating means or the first and second estimating algorithms, and the manipulation variable is generated according to the sliding mode control process, then the algorithm of the fuzzy inference process relative to the second estimating means or the second estimating algorithm should preferably comprise an algorithm for generating the value of a given linear function having as variable components time-series data of the output of the oxygen concentration sensor and determined depending on a switching function used in the sliding mode control process, and the value of the data of the output of the oxygen concentration sensor, as the parameters of an antecedent part of the algorithm of the fuzzy inference process, and also generating the data representing the estimated value as the parameters of a consequent part of the fuzzy inference process.
If the sliding mode control process is used to generate the sliding mode control process, then, according to the findings of the inventors of the present invention, it is possible to obtain relatively accurately the data representing the estimated values in a state where the output of the oxygen concentration sensor varies in a nonlinear range, by establishing the parameters of the antecedent part and consequent part of the fuzzy inference process as described above. As a result, the quick response of the control process of converging the output of the oxygen concentration sensor to the target value even in the state where the output of the oxygen concentration sensor varies in the nonlinear range.
When the given condition (the given condition which defines how to select or combine estimated values used to generate the manipulation variable) comprises the combination condition of the value of the given linear function determined depending on the switching function used in the sliding mode control process, and the value of the data of the output of the detecting means, the linear function relative to the antecedent part of the fuzzy inference process should preferably be the same as the linear function relative to the combination condition.
It is thus possible to appropriately to determine according to the above combination condition whether the output state of the exhaust gas sensor (oxygen concentration sensor) is a state for selecting, or attaching importance to, the estimated value according to the second estimating means or the second estimating algorithm to generate the manipulation variable, i.e., a state where the estimated value according to the fuzzy inference process is of better accuracy, or a state for selecting, or attaching importance to, the estimated value according to the first estimating means or the first estimating algorithm to generate the manipulation variable, i.e., a state where the estimated value based on the model of the exhaust system, etc. is of better accuracy. Consequently, the estimated values according to the first and second estimating means or the first and second estimating algorithms can be selected or combine in a manner to match the accuracy of the estimated values, and hence the manipulation variable can be generated more appropriately.
The algorithm of the fuzzy inference process should preferably comprise an algorithm constructed based on a min-max-center-of-gravity process, using a plurality of bar-shaped functions as membership functions relative to the parameters of the consequent part. The bar-shaped functions are functions having function values only at one value of their variables (parameters). The min-max-center-of-gravity process is a known process generally used in the fuzzy inference process.
In this manner, the algorithm of the fuzzy inference process can be constructed with ease, reducing the operating load for generating the data representing the estimated values.
In the apparatus for and the method of controlling the exhaust system (plant) with the oxygen concentration sensor used as the detecting means, if the first and second estimating means or the first and second estimating algorithms are employed, then it is preferable to detect the air-fuel ratio of the exhaust gas entering the catalytic converter with an air-fuel sensor, sequentially identify parameters to be set of the model of the exhaust system using the data of respective outputs of the air-fuel sensor and the oxygen concentration sensor, and generate the data representing the estimated value using at least the data of the respective outputs of the air-fuel sensor and the oxygen concentration sensor and identified values of the parameters of the model of the exhaust system.
By thus detecting the air-fuel ratio of the exhaust gas entering the catalytic converter with the air-fuel sensor, and sequentially identifying the parameters of the model of the exhaust system using the data of respective outputs of the air-fuel sensor and the oxygen concentration sensor which detects the oxygen concentration in the exhaust gas as the output of the exhaust system, the parameters of the model are identified on a real-time basis depending on the actual behavioral state, from time to time, of the exhaust system. Therefore, an error of the model of the exhaust system with respect to the actual behavior of the exhaust system is reduced to a minimum. As a consequence, the accuracy of the data of the estimated value according to the first estimating means or the first estimating algorithm in a state where the output of the oxygen concentration sensor varies in a substantially linear range can be increased, and the quick response of the control process of converging the output of the oxygen concentration sensor to the target value can be increased.
According to either one of the first and second aspects, the algorithm of the first estimating means (the first estimating algorithm) is capable of generating the data presenting the estimated value using both or one of the data of the output of the air-fuel ratio sensor and the data of the manipulation variable, the data of the output of the oxygen concentration sensor, and the parameters of the model of the exhaust system.
The model of the exhaust system which serves as a basis for the algorithm of the first estimating means (the first estimating algorithm) should preferably be constructed basically as a discrete-time system. In this case, the model of the exhaust system should preferably be a model expressing the data of the output of the oxygen concentration sensor in each given control cycle with the data of the output of the oxygen concentration sensor in a past control cycle prior to the given control cycle and the data representing the air-fuel ratio of the exhaust gas entering the catalytic converter in a control cycle prior to the dead time of the exhaust system (the data of the output of the air-fuel ratio sensor, the data of the manipulation variable, etc.)
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.